More and more functions in modern motor vehicles are being implemented using electrical components. This results in an ever higher demand for electrical power. To meet this demand, the efficiency of the generator system in the motor vehicle must be increased. Until now, p-n type diodes have ordinarily been used as Zener diodes in motor vehicle generator systems. The advantages of p-n type diodes are a low reverse current, on the one hand, and high robustness, on the other hand. The main disadvantage is a high forward voltage UF. At room temperature, current begins to flow only at UF=0.7V. Under normal operating conditions, e.g., a current density of 500 A/cm2, UF increases to over 1V, which means a not inconsiderable loss in generator efficiency.
Theoretically, Schottky diodes are available as an alternative. Schottky diodes have a much lower forward voltage than p-n type diodes, for example 0.5V to 0.6V at a high current density of, for example, 500 A/cm2. As majority carrier components, Schottky diodes also have advantages in rapid switching operation. However, Schottky diodes have not yet been used in motor vehicle generator systems. This circumstance is attributable to a number of important disadvantages of Schottky diodes: 1) higher reverse current compared to p-n type diodes; 2) great dependency of the reverse current on reverse voltage; and 3) poor robustness, in particular at high temperatures.
Ways to improve Schottky diodes have been proposed. One such method is the TMBS (Trench MOS barrier Schottky diode), see T. Sakai, et al., “Experimental Investigation of Dependence of Electrical Characteristics on Device Parameters in Trench MOS Barrier Schottky Diodes,” Proceedings of 1998 International Symposium on Power Semiconductors & ICs, Kyoto, pp. 293-296, and German Patent No. DE 69428996 A1.
As shown in FIG. 1, the TMBS is made of an n+-type substrate 1, an n-type epitaxial layer 2, at least two trenches 6 which are provided in n-type epitaxial layer 2 by etching, metal layers on the front of chip 4 as the anode electrode and on the back of chip 5 as the cathode electrode, and oxide layers 7 between trenches 6 and metal layer 4. From an electrical point of view, the TMBS is a combination of a MOS structure having a metal layer 4, oxide layers 7 and an n-type epitaxial layer 2 and a Schottky diode having Schottky barriers between metal layer 4 as the anode and n-type epitaxial layer 2 as the cathode. Currents flow in the forward direction through the mesa region between trenches 6. Trenches 6 themselves are not available for current flow. The effective area for current flow in the forward direction is therefore smaller in a TMBS than it is in a conventional planar Schottky diode.
The advantage of a TMBS lies in the reduction of reverse currents. In the reverse direction, space charge zones form in both the MOS structure and in the Schottky diode. The space charge zones expand as the voltage increases and converge in the middle of the region between adjacent trenches 6 at a voltage which is lower than the breakdown voltage of the TMBS. The Schottky effects responsible for high reverse currents are shielded thereby and the reverse currents reduced. This shielding effect is greatly dependent on the structural parameters of trench depth Dt, distance between trenches Wm, trench width Wt as well as on the thickness of oxide layer To (see FIG. 1).
Conventionally, the TMBS is manufactured by: providing trenches 6 by etching n-type epitaxial layer 2, growing oxide layer 7 and filling the trenches with conductive, doped layers made of polysilicon. Alternatively, the trenches may be filled with metal. The expansion of the space charge zones in the mesa region between trenches 6 is quasi one-dimensional, provided that trench depth Dt is much greater than distance between trenches Wm.
However, a disadvantage of the TMBS lies in the weakness of the MOS structure. At breakdown, very high electrical fields form within oxide layer 7 and in the direct vicinity of the oxide layer in n-type epitaxial layer 2. As a result, the MOS structure may become degraded due to the injection of “hot” charge carriers from n-type epitaxial layer 2 into oxide layer 7 and even be destroyed under certain operating conditions. Another disadvantage of the TMBS is its round or soft reverse characteristic. This means that, far before the actual breakdown, e.g., at a voltage=70% to 80% of the breakdown voltage, the reverse current already begins to rise substantially and is significantly higher than the reverse current at a lower voltage. This high reverse current in advance of the Schottky diode breakdown, may result in a high power loss, in particular at high temperatures, and also in thermal instability (thermal runaway) and failure of the component, due to positive, electrical/thermal feedback.